Can an O2 sensor cause an off idle stumble?

[neat]

Kind of got a strange one going on with the 87 TR.

It has been running great for awhile now, and then last night...

I had been driving for about an hour, and I stopped to get gas. I filled up, and took off. 2 blocks after filling up the car developed a stumble at idle. It would bounce around, not want to settle down, and stumble with anything less than about 10% throttle.

I have no check engine light.

I checked it with power logger and the only thing that looks odd to me is that the O2 sensor is reading .100-.300 at all times. I unplugged the O2 sensor and read the computer with PL again and the readings on the O2 sensor didn't change. That's telling me that either the O2 is crapped out, or the wiring between the O2 and the computer is messed up somewhere.

I checked for the obvious stuff, no vacuum leaks, the plugs look great, all cylinders are firing, and I didn't find any disconnected sensors or anything.

I'm running a turbo tweak chip, which has an open loop idle, so as far as I know the computer isn't paying attention to the O2 at idle. That being the case, I don't really see how the O2 could be causing the idle problem.

The IAC counts seem normal, about 30, and then it will rocket up to 130+ when the engine tries to stall.

The IAC is new, about 4 months old. I wonder if that could be causing the problem.

Fuel pressure is good, even during the stumble, about 42 PSI at idle with the vacuum line connected.

I'm wondering if the coil pack isn't the problem.

Any help is appreciated!

 

[turbojd]

Try and spray out the throttlebody and IAC with some carb cleaner and see if that helps.

 

[TurboBuRick]

Still running stock maf? See how it runs with it unplugged.

 

[neat]

I'll try unplugging the MAF, thanks!

I'm going to try swapping out the fuel as well. Since it happened within a couple blocks of filling up, maybe I got some really bad gas.

 

[yullose]

I noticed my biggest improvement when I performed the IAC reset procedure...

I have the PCV valve plugged-off... so the IAC needed to be reset so it would open enough to compensate for the missing air from the PCV. After doing the reset, it's much better... I'm seeing IAC numbers around 70 at idle.

Your problem almost sounds like a funky MAF sensor though.

 

[neat]

Still no luck with it.

The MAF readings look normal with power logger, so I hate to buy a MAF just to see. I tried unplugging it, and the car runs worse. I also tapped it with a wrench in several spots and nothing changes. Any other diagnostics I can do with the MAF sensor before plopping down the coin for a new one?

I swapped out the fuel, no change.

I bought an O2 sensor and swapped it out, no change.

I removed the chip from the ECM and re-installed, no change.

On my PL, the BLM at idle is 142. Is that normal? I've never paid attention to it before.

I'll dink with it some more on Sunday. Thanks guys!

 

[TurboBuRick]

The ecm is adding some fuel (blm 142) but it's not too out of range. If your 10 + or - of 130, you are ok. Sounds like a small vacuum leak. How does the maf hose look?

Can you post a short log?

 

[mygn276]

Mine as done this before for no reason i know. I advanced the cam sensor a tad and solved the problem. It wont hurt to try that.

 

[neat]

I think I found it. Last night I built and installed an EGR block off plate. The problem is 90% resolved. I re-used the gasket for the EGR valve, so it might still be leaking a little.

I ordered a new EGR gasket and valve today, they will be in tomorrow. I'm hoping that with the new valve and gasket, my problem will be gone.

 

[neat]

The EGR wasn't the problem. It turned out the screen in the MAF sensor had partially fallen out and was giving the MAF a fit. I fixed the screen and all is well now.

Thanks everyone.

 

[big lou]

Agageaa